The present invention relates to a bearing device suitable for use in head stack assemblies where a hard disk drive swing arm is moved in a swinging motion.
The devices shown in FIG. 2 and FIG. 3 are examples of a hard disk drive (HDD). In FIG. 2 and FIG. 3, HDD1 is roughly composed of a rectangular box-like container (base plate) 2, a spindle motor 3 disposed on the base plate, and a head stack assembly (hereinafter “HSA”) 6 with magnetic heads 5 which write information in specified locations on a magnetic disk 4, rotated by spindle motor 3, and reads data from any location.
HSA 6 contains a tubular part 8 equipped with a swing arm 7 and magnetic heads 5 at the tip. In addition, HSA includes a shaft 9 containing an inner ring attached to base plate 2. Shaft 9 is composed of a bearing device 10 which supports swing arm 7 so it can swing about shaft 9 and the drive part for driving swing arm 7. Shaft 9, as shown in FIG. 1, typically comprises a rectangular shaft body 9a and a flange 9b, formed at one end of the shaft body 9a. Flange 9b is attached to base plate 2.
FIG. 4 shows a conventional bearing device 010. As shown in FIG. 4, this bearing device 010, roughly comprises two (hereinafter “first” and “second”) single row deep groove ball bearings (hereinafter “ball bearings”) 012 and 013 installed on shaft 09. A sleeve 014 is disposed outside of the outer rings (hereinafter, “first and second outer rings”) 012b and 013b of the first and second ball bearings 012 and 013, with one end of the inner ring 012a of the first ball bearing 012 (hereinafter, “the first inner ring”, and the inner ring of the second ball bearing will hereinafter be referred to as “the second inner ring”) being in contact with flange 09b. 
Sleeve 014 typically comprises tubular sleeve body 014a and a flange 014b, formed on one end of sleeve body 014a. Sleeve 014 corresponds to the outer edge of first outer ring 012b, and it is disposed outside of the first and second ball bearings 012 and 013. In addition, the end surface of flange 014b and the outer edge surface of first outer ring 012b are arranged on a single surface. The end surface of sleeve body 014a and the outer edge surface of second outer ring 013b are also arranged on a single surface.
The width dimensions of the first and second outer rings 012b and 013b are set to the same dimension A, and the width dimensions of the first and second inner rings 012a and 013a are also set to the same dimension B, such that A>B. In this case, the reduced setting for the width dimension B of the first and second inner rings 012a and 013a is done so that it only shortens the equal distance (A−B/2) from each of the ends of the first and second outer rings 012b and 013b. 
This distance (A−B/2) is greater than the amount δ of a one-sided rattle of each axial orientation (the axial orientation of the first ball bearing 012, the axial orientation of the second ball bearing 013), and it is a dimensional difference capable of preventing the production of an axial one-sided rattle of each ball bearing which imparts a preload to one end of each double-end part of the first and second inner rings 012a and 013a (the double-end part of the first inner ring 012a, the double-end part of the second inner ring 013a). For example, in FIG. 4, it is a dimensional difference capable of preventing the production of an axial one-sided rattle of the second ball bearing 013, which imparts a preload in direction C to the outer end of the second inner ring 013a. 
Generally, the “axial rattle” of the ball bearing is the sum of the previously-set dimensions of the axial clearance of the ball bearing, and the axial relative dimensions of the outer ring and inner ring, determined by the elastic deformation of the ball bearing produced by the application of a set preload. Because the natural state is for the rolling element to be supported by both rolling grooves, whose point contact is the center of the rolling groove of the outer ring and the center of the rolling groove of the inner ring, a one-sided rattle is formed by pushing one end of either the inner ring or the outer ring, and an opposite-side rattle is formed by pushing the other end. The total amount of the “axial rattle” of the ball bearing is the sum of the amounts of both these rattles.
The first and second inner ring rolling grooves 012d and 013d of the first and second inner rings 012a and 013a are formed at the center of the width of the first and second inner rings 012a and 013a. Therefore, centered on the centers of the first and second inner ring rolling grooves 012d and 013d, the width dimensions of both the first and second inner rings 012a and 013a are both B/2 and equal.
The first and second ball bearings 012 and 013 are installed on the shaft 09 so that they touch the first and second outer rings 012b and 013b, and between the first and second inner rings 012a and 013a, in the state prior to applying a preload to the second inner ring 013a, a space S with a maximum (A−B) length is formed. In addition, the distance P between the first and second rolling elements 012c and 013c of the first and second ball bearings 012 and 013 is equal to A.
In this bearing device 010, with first and second outer rings 012b and 013b attached, they are fixed to the inner face of the sleeve body 014a with an adhesive. First inner ring 012a is installed on shaft 09 and fixed with an adhesive. Second inner ring 013a is slidably installed on shaft 09. The outer end of second inner ring 013a applies a preload in the direction of the arrows C in FIG. 4, and while such a preload is applied, the second inner ring 013a is fixed to the shaft 09 with an adhesive. This structure eliminates the axial rattle so that the desired precision and rigidity of the bearing device 010 are maintained.
The size (A−B) of space S is set so that it is greater than 2δ of the rattles of the bearing device 010 (the sum of the amount δ of the axial one-sided rattle of the first ball bearing 012 and of the amount δ of the axial one-sided rattle of the second ball bearing 013), and when a preload is applied to the second inner ring 013a, the amount of preload can be adjusted over a wide range.
In conventional bearing devices when the width dimension of the first and second outer rings and the width dimensions of the first and second inner rings are identical, elimination of the rattling of the bearing device when a preload is applied, relied on methods such as shaping space S between the first and second inner rings so that an annular projection separates the first and second outer rings in the inner surface of the sleeve or creating an annular space, formed by a separate member, between the first and second outer rings. But in bearing device 010, there is no need to use such an annular projection of the sleeve inner surface or an annular space created by a separate member, and to this extent, the width dimensions of the overall body of the bearing device 010 can be reduced. Therefore, the thickness of swing arm 7 support can be reduced and the HDD 1 can be made thinner.
Moreover, in bearing device 010, in the state prior to the application of a preload to the second inner ring 013a, the centers of the first and second outer ring grooves 012e and 013e of the first and second outer rings 012b and 013b, the centers of the first and second inner ring grooves 012d and 013d of the first and second inner rings 012a and 013a, and the centers of the first and second rolling elements 012c and 013c are on a single plane. Because the first and second ball bearings 012 and 013 have a symmetrical structure with respect to this plane, when the first and second ball bearings 012 and 013 are installed on the shaft 09 and the bearing device 010 is assembled, there is no need to control the arrangement of the first and second ball bearings 012 and 013. Therefore, the production efficiency can be increased.
Bearing device 010 is very useful for PC card type ultra-thin hard disk drive devices, for which recently there has been a particularly strong demand.
In another conventional embodiment, bearing device 010 can also be constructed without sleeve 014 as seen in FIG. 5. In bearing device 010 of FIG. 5, the first and second outer rings 012b and 013b are in contact with each other. First inner ring 012a is installed on shaft 09 and fixed with an adhesive, while second inner ring 013a is slidably installed on the shaft 09. A preload is then applied to the outer end of the second inner ring 013a in the direction of the arrows C in FIG. 5. While such a preload is applied, second inner ring 013a is fixed to shaft 09 with an adhesive, thus eliminating the axial rattle so that the desired precision and rigidity of bearing device 10 are maintained.
Bearing device 010 is constructed similar to the bearing device shown in FIG. 4. Space S is maintained between the first and second inner rings 012a and 013a. The annular protrusion of the sleeve inner surface and the annular space formed by a separate member, which have so far been necessary, become redundant, and to such an extent, the width orientation dimensions of the body of the bearing device 010 can be reduced. Consequentially, the thickness of swing arm 7 support can be reduced and the HDD 1 can be made thinner. Moreover, when the first and second ball bearings 012 and 013 are installed on the shaft 09 and the bearing device 010 is assembled, there is no need for control of the arrangement of the first and second ball bearings 012 and 013, and the production efficiency can thus be increased.